U.S. patent application number 10/355736 was filed with the patent office on 2004-08-05 for baseline compensating method and camera used in millimeter wave imaging.
Invention is credited to Huguenin, G. Richard, Vaidya, Nitin M..
Application Number | 20040149909 10/355736 |
Document ID | / |
Family ID | 32770610 |
Filed Date | 2004-08-05 |
United States Patent
Application |
20040149909 |
Kind Code |
A1 |
Vaidya, Nitin M. ; et
al. |
August 5, 2004 |
Baseline compensating method and camera used in millimeter wave
imaging
Abstract
Scene-independent baseline signals in output value signals from
radiometer or receiver channels used in millimeter wave imaging are
eliminated or reduced and an improved image is composed. The
scene-independent baseline signals are believed to result from a
standing wave which is established between an antenna of the
channel and a movable scanning element which scans radiant energy
from the scene into each channel. The movable scanning element
introduces changes in geometry which change the characteristics of
the baseline signals depending upon the position of the movable
scanning element. The baseline signals are measured by viewing a
scene of uniform brightness temperature, and the baseline signal
contribution is subtracted from the output value signals from each
channel. The baseline compensated output signals are used to
compose an image with better contrast.
Inventors: |
Vaidya, Nitin M.;
(Belchertown, MA) ; Huguenin, G. Richard; (Marion,
MA) |
Correspondence
Address: |
JOHN R LEY, LCC
5299 DTC BLVD, SUITE 610
GREENWOOD VILLAGE
CO
80111
US
|
Family ID: |
32770610 |
Appl. No.: |
10/355736 |
Filed: |
January 31, 2003 |
Current U.S.
Class: |
250/338.1 |
Current CPC
Class: |
G01S 13/887 20130101;
H01Q 13/085 20130101; G01J 5/52 20130101 |
Class at
Publication: |
250/338.1 |
International
Class: |
G01J 005/00 |
Claims
What is claimed is:
1. A method used in millimeter wave imaging to reduce a
scene-independent baseline signal component of an output value
signal from a channel into which radiant energy emanated from a
scene is scanned by a movable scanning element, comprising:
obtaining a magnitude of the baseline signal at each position of
the movable scanning element; and subtracting the magnitude of the
baseline signal at each position of the movable scanning element
from the output value signal of the channel derived from radiant
energy scanned into the channel from a scene of non-uniform
brightness at corresponding positions of the movable scanning
element.
2. A method as defined in claim 1, further comprising: measuring a
magnitude of the baseline signal at each position scanning radiant
energy into the channel from a scene of uniform brightness.
3. A method as defined in claim 2, further comprising: composing an
image of the scene from a plurality of output value signals from
which the baseline signal has been subtracted.
4. A method as defined in claim 2, further comprising: composing an
image of the scene based on output value signals obtained at each
position of the movable scanning element from which the baseline
signals at those corresponding positions have been subtracted.
5. A method as defined in claim 4, further comprising: composing
the image from output value signals obtained from a plurality of
channels into which the radiant energy from the scene has been
scanned.
6. A method as defined in claim 1, further comprising: using a
plurality of channels in the millimeter wave imaging; creating
baseline-compensated output signals for each channel at each
position of the movable scanning element by subtracting the
baseline signal from the output value signals at the corresponding
positions of that channel; and composing an image of the scene by
adding a plurality of baseline-compensated output signals from a
plurality of channels.
7. A method as defined in claim 6, further comprising: composing
the image with a plurality of pixels, each pixel corresponding to a
point in the scene; scanning the radiant energy emanating from each
point in the scene into a plurality of different channels; and
composing an intensity of each pixel of the image from the
baseline-compensated output signals from each channel into which
the radiant energy emanating from the corresponding point in the
scene is scanned.
8. A method as defined in claim 7, further comprising: composing
the intensity of each pixel of the image by adding the
baseline-compensated output signals from each channel into which
the radiant energy emanating from the corresponding point in the
scene is scanned.
9. A method as defined in claim 8, further comprising: weighting
each baseline-compensated output signal by a different amount prior
to adding the baseline-compensated output signals from each of the
plurality of channels to compose the intensity of each pixel.
10. A method as defined in claim 9, further comprising: weighting
each baseline-compensated output signal by a predetermined
weighting factor related to an amount of noise in each output value
signal from each channel.
11. A method as defined in claim 10, further comprising: using as
the weighting factor a reciprocal of the standard deviation of the
amount of noise in each output value signal from each channel.
12. A method as defined in claim 11, further comprising: measuring
a magnitude of noise from each channel into which radiant energy is
scanned from a scene of uniform brightness; and computing the
standard deviation of the output value signals from 5 each channel
based on the measured magnitude of noise.
13. A method as defined in claim 11, further comprising:
normalizing the weighted baseline-compensated output signals from
each channel on the basis of a flat fielding response of the
channels.
14. A method as defined in claim 13, further comprising:
normalizing the weighted baseline-compensated output signals from
each channel by a gain factor related to an amplification
capability of each channel.
15. A method as defined in claim 14, further comprising: measuring
the amplification capability of that channel in response to
scanning radiant energy from two uniform scenes of different and
known brightness temperatures into that channel; and establishing
the gain factor of each channel in response to the measured
amplification capability.
16. A method as defined in claim 13, further comprising:
normalizing the weighted baseline-compensated output signals from
each channel by a normalizing factor which is related to a drift in
offset of the output value signals from each channel.
17. A method as defined in claim 16, further comprising:
calculating the normalizing factor from information defining the
individual gain and offset characteristics of each of the
channels.
18. A method as defined in claim 17, further comprising:
calculating the normalizing factor with each scan of radiant energy
from the entire scene into the plurality of channels.
19. A method as defined in claim 17, further comprising:
calculating the normalizing factor based on a consistency condition
that an unknown mean scan temperature encountered by each channel
into which radiant energy from a portion of the scene is scanned is
equal to the average of the intensities of those pixels to which
the baseline-compensated output signals from that channel
contributes; and calculating the average of pixel intensities based
on an expression for the average pixel intensities in terms of the
unknown mean scan temperatures of the channels.
20. A method as defined in claim 11, further comprising:
normalizing the weighted baseline-compensated output signals from
each channel on the basis that each different channel observes a
different mean scan brightness temperature from radiant energy
scanned from a portion of the scene than the mean brightness
temperature of the entire scene.
21. A method as defined in claim 11, further comprising: forming an
intermediate image by adding the weighted baseline-compensated
output signals at each pixel of the intermediate image; and
transforming the intermediate image into a final image by adjusting
the weighted baseline-compensated output signals from each channel
by a normalizing factor related to gain and drift in offset of the
output value signals from each channel.
22. A method as defined in claim 1 used in passive millimeter wave
imaging.
23. A millimeter wave camera implementing the method defined in
claim 11, comprising: a signal processor for performing the actions
and calculations to subtract the baseline signal component from the
output value signals to establish the baseline-compensated output
signals, to weight the value of each baseline-compensated output
signal, to normalize the baseline-compensated weighted output
signals, and to compose the intensity of each pixel of the image by
adding the values of the normalized, baseline-compensated and
weighted signals; the signal processor performing the actions and
calculations in response to the output value signals from each
channel from scanning a scene having a non-uniform brightness
temperature, to gain information describing an amplification
capability of each channel, and to deviations in response
characteristics of each channel in response to scanning a scene of
uniform brightness temperature.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is related to U.S. patent applications for
an invention for an Offset Drift Compensating Flat Fielding Method
and Camera Used in Millimeter Wave Imaging Serial No. (246.301),
and for a Weighted Noise Compensating Method and Camera Used in
Millimeter Wave Imaging, Serial No. (246.303), both filed
concurrently herewith and assigned to the assignee of the present
application. The subject matter of these related applications are
incorporated herein by this reference.
FIELD OF THE INVENTION
[0002] This invention generally relates to millimeter wave imaging.
Millimeter wave imaging involves creating an image of a scene from
millimeter wavelength energy signals emanating from the scene. More
particularly, the present invention relates to a new and improved
method and camera which compensates for a baseline signal anomaly
associated with each of a plurality of radiometer channels or
receiver channels which receive and detect millimeter wavelength
energy signals emanating from a scene and scanned into each channel
by a movable scanning element. The anomalous baseline signal arises
because of standing waves between the channel and the movable
scanning element. By subtracting the baseline signal from output
value signals from each channel, the response of each channel to
the millimeter wavelength energy emanated from the scene more
accurately represents the scene. The contrast and quality of an
image composed by summing the output value signals from the
channels is improved.
BACKGROUND OF THE INVENTION
[0003] Millimeter waves are electromagnetic radiation characterized
by wavelengths in the range of from 1 to 10 millimeters and having
corresponding frequencies in the range of 300 GHz to 30 GHz.
Millimeter waves have the capability of passing through some types
of objects which would stop or significantly attenuate the
transmission of electromagnetic radiation of other wavelengths and
frequencies. For example, millimeter waves pass through clothing
with only moderate attenuation, pass through doors and walls, are
capable of penetrating slight depths of soil, and are not obscured
or adversely influenced by fog, cloud cover and some other types of
visually-obscuring meteorological conditions. Because of these
properties, millimeter wave imaging has been employed to detect
contraband and weapons concealed beneath clothing of an individual,
to alert law enforcement authorities of the location of individuals
and objects within the interior of a room or building prior to
executing search warrant raids, to detect the presence and location
of buried land mines, and for landing and takeoff guidance for
aircraft when meteorological conditions obscure runways, among many
other things.
[0004] According to known laws of physics, the amount or intensity
of electromagnetic energy emitted by an object is proportional to
its physical temperature measured in degrees Kelvin. The radiation
originates from thermally-induced charged-particle accelerations,
subatomic particle interactions and other quantum effects. These
quantum effects account for a distribution of radiation throughout
a broad spectrum of frequencies, as recognized by Planck's Law.
Consequently, it is typical to characterize the amount of energy
emanating from a point or object in a scene by its apparent
brightness temperature.
[0005] The energy emanating from a point or object in the scene
results from emission and reflection. Emission and reflection are
related to one another such that highly emissive objects are only
slightly reflective, and highly reflective objects are only
slightly emissive. Passive millimeter wave imaging creates an image
from both the emitted and the reflected electromagnetic energy.
Active millimeter wave imaging also relies on energy emission and
reflection, but enhances the energy content in a scene by
illuminating the scene with added energy. The added energy
increases the contrast or distinction in energy emanated from
different points within the scene, primarily by increasing the
reflected energy. Because passive millimeter wave imaging relies on
the inherent natural energy emanating from the objects and the
background in the scene, and such inherent natural energy is
generally less than the amount of energy resulting from actively
illuminating the scene with added energy, it is typically more
difficult to create an image passively.
[0006] In some scenes, the distinction between the brightness
temperature of an object and the brightness temperature of the
background is relatively small. Slight differences in the
brightness temperature of the objects and the background increase
the difficulty of detecting those energy differences with enough
distinction to create images with good contrast and resolution
relative to the background. Inadequate contrast, resulting from an
inability to detect relatively small differences in radiated energy
from point to point within the scene, degrades the quality of the
image. The ability to form good millimeter wave images is therefore
directly related to the ability to recognize relatively small
differences in the amount of millimeter wave energy emanated from
different points within the scene, which is particularly important
in passive millimeter wave imaging because of the relatively small
differences in energy emanated from objects in the scene.
[0007] Millimeter wave imaging is further complicated by the fact
that millimeter wave energy constitutes only a very small band or
part of the spectrum of energy emitted by a body. The
temperature-related quantum effects result in an energy
distribution throughout a wide spectrum of frequencies. For
millimeter wave imaging, only the frequency spectrum of radiation
within the millimeter wavelength (30-300 GHz) is examined.
Moreover, the typical millimeter wavelength frequency band used in
millimeter wave imaging is even further restricted, for example, at
94.+-.2 GHz. The amount of energy available is generally related to
the bandwidth. Consequently, the limited bandwidth also reduces the
amount of energy available to be detected for use in creating
millimeter wavelength images.
[0008] Further complications arise from the noise-like origin of
the millimeter wavelength energy which is detected to create the
images. The thermally induced quantum effects result in a
significant variations in frequency distribution and intensity of
the emitted energy, thereby causing the radiated energy to have
random characteristics similar, to noise. In the usual sense, noise
is considered as a factor which contaminates or derogates an
otherwise pure signal. The relatively pure nature of the underlying
signal assists in distinguishing the corrupting noise and
eliminating its effects, in typical signal processing. However,
there is no underlying pure signal in passive millimeter wave
imaging, due to the thermally induced and random quantum effects
which create the emitted radiation. Consequently, it is necessary
to rely on a primary noise-like signal for the information to
create the image, and to attempt to eliminate or reduce the effects
of other noise-like signals that have the potential to obscure the
desired information from a primary signal. Thus, distinguishing the
desired information carried by a noise-like signal from spurious
and derogating noise-like signals of similar characteristics is a
significant challenge in millimeter wave imaging.
[0009] The noise-like origin and characteristics of natural
millimeter wave radiation, the limited bandwidth of energy within
the millimeter wavelength spectrum from which to form the image,
the relatively small differences in brightness temperature of the
object in a scene compared to its background, and many other
factors, have indicated a capability to enhance millimeter wave
imaging by using multiple channels (radiometer channels are
typically used for passive imaging and receiver channels are
typically used for radar and most types of active imaging, although
radiometer channels may be used in certain instances for non-radar
active imaging), arranged in a focal plane array and scanning the
energy emanating from the scene into the multiple channels. The
channels convert the received or scanned-in radiant energy into
electrical output value signals or samples. The multiple output
values or samples from multiple different channels scanning each
point are added together to create a pixel in the image which
corresponds to that point in the scene. Each pixel has an intensity
which is derived from adding the multiple samples.
[0010] One disadvantage of using multiple radiometer channels or
receiver channels to obtain the multiple samples to be added
together is that each channel has its own individual and particular
response characteristics. In response to viewing exactly the same
point having one brightness temperature, each channel creates a
slightly different output value. When the slightly different
samples from the multiple channels are combined to create each
pixel, the intensity of the pixel does not faithfully represent the
brightness temperature of the corresponding point in the scene.
Adding signals which are slightly different, even when those
signals originate from a single point in the scene with a uniform
brightness temperature, results in slight derogation in contrast of
the image. Such image derogation is not related to the energy
content of the scene, but is related to the slightly different
characteristics of the channels used to obtain the samples.
Moreover because of the scanning effect, the anomalous effects
introduced by the individual and different characteristics of each
channel are distributed among various different pixels in the
image, thereby decreasing the contrast and the quality of the
image.
[0011] Despite careful efforts to make each radiometer and receiver
channel exactly the same, each channel has its own unique gain,
offset and noise temperature and response characteristics. Gain
refers to the capability of the channel to amplify input signals it
receives. Each channel characteristically amplifies a known
constant input signal by a slightly different amount. Offset refers
to a characteristic output signal level of the channel in response
to a known input signal. The output signal level from each channel
will be slightly different in magnitude in response to a known
uniform input signal. The noise temperature characteristics of a
channel relate primarily to electrical imperfections of components
used in the channel, as opposed to the physical temperature of the
channel itself. The noise temperature is extremely high relative to
physical thermal temperature, and each channel has a significantly
different noise temperature even when the physical temperature of
the channels is maintained uniform.
[0012] To counteract the effects of the individual response
characteristics of each channel, it is traditional to position a
mechanical chopper in the optical path between the scene energy and
the channels. The chopper periodically and rapidly introduces a
known uniform brightness element, such as a black body, into the
optical path, and each channel is quickly readjusted while the
uniform brightness element is momentarily inserted in its optical
path. The use of such choppers, and the necessity to quickly
readjust each channel while still measuring radiation from the
scene, greatly complicates the imaging process and the equipment
necessary to perform the imaging.
[0013] To avoid the use of choppers, efforts have been made in the
past to normalize the gain response characteristics of each
channel. Normalization involves dividing the output response of
each channel by the gain of the channel. In this manner, the
response of each channel is gain normalized, so that when the
samples from the channels are added, their contributions are of
uniform relativity based on gain. While gain normalization has
enhanced the quality of the image, gain normalization has not
eliminated image anomalies arising because of the particular
differences in offset and noise temperature characteristics of the
channels. Moreover, the typical type of gain normalization employed
in the prior art has been discovered not to account adequately for
all variations in gain among the different channels.
SUMMARY OF THE INVENTION
[0014] The present invention involves a new and improved method of
reducing or eliminating scene-independent baseline signals in
output value signals from radiometer channels or receiver channels
into which radiant energy from a scene is scanned by a movable
scanning element during millimeter wave imaging. The present
invention recognizes a probable cause of these scene-independent
baseline signals, and describes a technique for measuring the
baseline signals to compensate the output value signals for the
purpose of composing a better quality image.
[0015] It is believed that the scene-independent baseline signals
result from a standing wave which is established between an antenna
of the channel and the movable scanning element. Because the
movable scanning element introduces changes in geometry of the
optical path through which radiant energy from the scene is scanned
into the channels, the characteristics of the standing wave vary in
relation to the position of the movable scanning element.
Recognizing the dependency on the position of the movable scanning
element allows the influence of the baseline signal to be measured
at each position of the movable scanning element, so that the value
of the baseline signal may thereafter be subtracted from the output
value signals obtained directly from the channels. The result of
the subtraction is a baseline-compensated output signal which more
faithfully and accurately represents the brightness temperatures of
points within the scene. Consequently, the characteristics of the
scene may be more accurately imaged with contrast. Alternatively,
less difference in radiated energy from different points within the
scene is effective in creating images.
[0016] It is believed that scene-independent baseline signals of
the kind that vary in relation to the position of a movable
scanning element have not previously been recognized in the field
of millimeter wave imaging. In general, the invention pertains to
reducing a scene-independent baseline signal component of an output
value signal from a channel into which radiant energy emanated from
a scene is scanned by a movable scanning element, in the context of
millimeter wave imaging. A method of the invention involves
obtaining a magnitude of the baseline signal at each position of
the movable scanning element, and subtracting the magnitude of the
baseline signal at each position of the movable scanning element
from the output value signal of the channel derived from radiant
energy scanned into the channel from a scene of non-uniform
brightness at corresponding positions of the movable scanning
element. Preferably, the magnitude of the baseline signal is
obtained by measuring at each position based on scanning radiant
energy from a scene of uniform brightness into the channel.
[0017] Other preferable aspects of the invention involve composing
an image of the scene from a plurality of output value signals from
which the baseline signals have been subtracted, each of which is a
baseline-compensated output signal. The image is preferably
composed from the baseline-compensated output signals from a
plurality of channels, by adding a plurality of the
baseline-compensated output signals to create the intensity of each
pixel of the image. Each pixel of the image corresponds to a point
in the scene. Radiant energy emanating from each point in the scene
is scanned into a plurality of different channels, and the
intensity of each pixel is composed by adding the
baseline-compensated output signals from each of the channels that
receive radiant energy scanned from the corresponding point in the
scene.
[0018] In composing the image, the baseline-compensating output
signals are preferably weighted by a different amount prior to
adding them to create the intensity for each pixel, and the
weighting factor applied to each baseline-compensated output signal
is related to an amount of noise in each output value signal from
each channel. Preferably the weighting factor is a reciprocal of
the standard deviation of the amount of noise in each output value
signal from each channel. The weighting factor and the standard
deviation are preferably obtained by measuring noise, from which
the weighting factor and the deviation are computed.
[0019] A further preferable step in composing the image involves
normalizing the weighted baseline-compensated output signals from
each channel on the basis of a flat fielding response of the
channels. The normalization involves compensation for the different
gain of each channel and the different drift in offset of the
output value signals from each channel. The gain characteristics of
each channel are preferably measured, while the different offset
characteristics of each channel are preferably calculated based on
a consistency condition that an unknown mean scan temperature
encountered by each channel into which radiant energy from a
portion of the scene is scanned is equal to the average of the
intensities of those pixels to which the baseline-compensated
output signals from that channel contributes, where the average of
pixel intensities is calculated based on an expression for the
average pixel intensities in terms of the unknown mean scan
temperatures of the channels. The basis for the calculation is that
each channel observes a different mean scan brightness temperature
from a portion of the scene from which radiant energy is directed
to the channel in comparison to the mean brightness temperature of
the entire scene.
[0020] Another aspect of the present invention pertains to a
millimeter wave imaging camera implementing the methodology
described above. The millimeter wave imaging camera comprises a
signal processor for performing the actions and calculations to
subtract the baseline signal component from the output value
signals to establish the baseline-compensated output signals, to
weight the value of each baseline-compensated output signal, to
normalize the baseline-compensated weighted output signals, and to
compose the intensity of each pixel of the image by adding the
values of the normalized, baseline-compensated and weighted
signals.
[0021] A more complete appreciation of the scope of the present
invention and the manner in which it achieves the above-noted and
other improvements can be obtained by reference to the following
detailed description of presently preferred embodiments taken in
connection with the accompanying drawings, which are briefly
summarized below, and by reference to the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] FIG. 1 is a block and schematic diagram of a millimeter wave
imaging camera which incorporates and illustrates features of the
present invention.
[0023] FIG. 2 is a partial perspective view of a number of
individual radiometer or receiver channels of a focal plane array
in the camera shown in FIG. 1.
[0024] FIG. 3 is an electrical component block diagram of a single
radiometer channel, of the type which might be used as one of the
channels of the focal plane array shown in FIGS. 1 and 2.
[0025] FIG. 4 illustrates five separate scanning paths of five
separate channels created by a movable scanning element of the
camera shown in FIG. 1.
[0026] FIG. 5 is a more complete version of FIG. 4, illustrating
the overlapping nature of the scanning paths of sixty-four separate
channels of a focal plane array.
[0027] FIG. 6 is a flow chart of the actions and computations
performed by the camera shown in FIG. 1 to compose an image in
accordance with the methodology of various aspects of the present
invention.
TERMINOLOGY
[0028] In the following Detailed Description, certain nomenclature
and symbols are used to describe the present invention. Although
the terms are also described in the Detailed Description, the
following list presents many of those terms for reference
purposes.
1 Symbol Definition A The NxN matrix used in the derivation of the
solution for the .theta..sub.i. A.sub.i The i.sup.th column vector
of matrix A. A' The Nx(N - 1) matrix that results from deleting the
N.sup.th column vector of matrix A. (A').sub.inv The (N - 1)
.times. N matrix used in computing .theta..sub.i. b The N x 1
column vector used in computing .theta..sub.i. B.sub.ik The
baseline signal of channel number i at observation position k. (1
.ltoreq. i .ltoreq. N, 1 .ltoreq. k < L) .beta. The
pre-detection bandwidth of a channel. C(m) The contribution counts
to each pixel m in the composed image for 1 .ltoreq. m .ltoreq. M.
g.sub.i The gain or amplification of channel number i. (1 .ltoreq.
i .ltoreq. N) i The designation of one channel in the focal plane
array. I(m) The image intensity at pixel number m of the display.
(1 .ltoreq. m .ltoreq. M) I'(m) The image intensity at pixel number
m of the display of an intermediate image formed during the
composition of a final image using a drift compensated flat
fielding technique described below. k The observation position of
the movable scanning element, such as the wedge shaped element,
where an observation of the output value signals from each channel
of the focal plane array is sampled. L The number of observation
positions of the movable scanning element where a channel output
value signal or observation is sampled or obtained. Also, the
number of subframes used to form a complete image of the scene,
with each subframe containing N number of channel output value
signals u at each observation position. M The selected maximum
number of pixels which constitute the display. m The designation
for each pixel of the display which contributes to the image
composed. n The number of a scan number. N The number of channels
in the focal plane array. Also, the number of simultaneous linear
equations expressing relationships between the unknown mean
relative scan brightness temperatures .theta..sub.1, .theta..sub.2,
. . ., .theta..sub.N. u The uncompensated output value signal from
a channel, each also referred to as an observation. s.sub.i The
scene brightness temperature encountered by channel i.
T.sub.i.sup.R The noise temperature for channel i. u.sub.i The
uncompensated output value signal u from a channel i. u.sub.ik(n)
The uncompensated output value signal u from a channel i at
rotational position k in scan number n. v The number of complete
scans (e.g., revolutions for the rotating wedge shaped element)
which creates sufficient data for forming values of the baseline
signal characteristics for each channel. u.sub.o,i The offset value
of channel i. x.sub.ik The channel output value signal or
observation sampled or obtained from the channel i at observation
position k, after baseline subtraction. (1 .ltoreq. i .ltoreq. N, 1
.ltoreq. k .ltoreq. L) 1 x ^ ik The channel output value signal or
observation x.sub.ik from the channel i of the focal plane array at
the observation position k of the movable scanning element after
normalization using a flat fielding technique. (1 .ltoreq. i
.ltoreq. N, 1 .ltoreq. k .ltoreq. L) 2 x _ i 3 The average 1 L l =
1 L x il of the output value signals or observations from channel i
over one complete scan (all L observation positions) by the movable
scanning element. (1 .ltoreq. i .ltoreq. N) .sigma..sub.i The
standard deviation of the random noise signal produced by the
channel number i, (1 .ltoreq. i .ltoreq. N), when that channel
views a scene of constant brightness temperature. 4 ^ i The
standard deviation of the normalized version of the random noise
signal produced by the channel number i, (1 .ltoreq. i .ltoreq. N),
when that channel views a scene of constant brightness temperature.
5 i V The standard deviation of the output value signal of the
channel i when viewing a general scene i.e. a scene with an
arbitrary brightness temperature distribution. .sigma..sup.Backend
The backend noise standard deviation of a channel. .tau. The time
constant associated with a post-detection bandwidth of a channel.
.theta..sub.i The mean relative scan brightness temperature seen by
each channel i, (1 .ltoreq. i .ltoreq. N) in the course of a
complete scan. .delta..sub.ikm The binary quantity that has value 1
if the output value signal from channel i at observation position k
contributes to pixel number m, and it has the value 0 otherwise.
.delta..sub.ikm specifies the scanning pattern or scanning
trajectory of each channel of the focal plane array, and are known
quantities for each channel for 1 .ltoreq. i .ltoreq. N, 1 .ltoreq.
k .ltoreq. L, and 1 .ltoreq. m .ltoreq. M.
DETAILED DESCRIPTION
[0029] A camera 10, shown in FIG. 1, incorporates and operates in
accordance with the various aspects of the present invention. The
camera 10 includes a movable scanning element 12 which receives
emitted and reflected radiation energy emanated from a scene 14.
The scene 14 is formed by points 16, and each of the points 16 is
either part of an object 18 within the scene 14 or part of a
background 20 within the scene 14. The points 16 of the scene 14
emanate radiation energy with an intensity which is related to the
brightness temperature of that point. The emanated radiation energy
includes energy in a millimeter wavelength frequency band which is
emitted and reflected from the objects 18 and the background
20.
[0030] The movable scanning element 12 receives emanated energy
from the scene 14, and directs or steers that energy through an
optical element 22 to a focal plane array 24 of channels 26. Each
channel 26 will typically be a radiometer channel when the camera
10 is used in passive imaging, and each channel 26 will typically
be a receiver channel when the camera 10 is used in active imaging,
although it may be possible that either type of channel may be used
for either type of imaging. The individual channels 26 of the focal
plane array 24 are shown in FIG. 2 and an exemplary radiometer
channel 26' is shown in FIG. 3. As shown in FIG. 1, an optical path
28 extends from the scene 14, through the movable scanning element
12, to the optical element 22 and to the focal plane array 24.
[0031] As shown in FIG. 2, the focal plane array 24 is formed by a
multiplicity of individual channels 26, arranged in a
two-dimensional array to receive radiation energy. Each channel 26
responds to energy received by an antenna 30 of each channel 26, as
is also shown in FIG. 3. Each antenna 30 preferably takes the form
of a conventional endfire traveling wave slot antenna. Each antenna
30 is preferably formed by two metallic elements 32 and 34 that
remain on a dielectric support substrate 36 after sheet metal (not
shown) which was originally attached to the substrate 36 has been
photolithographically etched away to leave the remaining elements
32 and 34. In this manner, a plurality of channels 26 can be formed
in parallel on a single support substrate 36, and the focal plane
array 24 may be formed by stacking or positioning a number of the
support substrates 36, each with a plurality of parallel channels
26, in parallel adjoining relationship with one another, as shown
in FIG. 2.
[0032] The shape of the metallic elements 32 and 34 establishes a
cavity 38 into which the radiation from the scene 14 (FIG. 1) is
received. Each channel 26 establishes an output value signal based
on the amount of radiation received by the antenna 30. The shape of
the cavity 38 establishes a field of view for each channel 26. The
field of view is that angular spatial volume from which radiation
energy is received by the antenna 30. The inherent field of view of
each antenna 30 is defined by a relatively slight angle of
divergence relative an axis (not shown) between symmetrical parts
of the cavity 36 and parallel to the metallic elements 32 and
34.
[0033] The inherent field of view of each channel 26 causes the
vast majority of radiation energy to be received within the
relatively slight angle of the field of view of each antenna 30.
Because the natural field of view of each antenna 30 is limited in
this manner, the natural field of view of each channel 26 is not
usually sufficient to receive energy from the entire scene 14 (FIG.
1), even when multiple channels 26 are employed in the array 24 of
the camera 10. Consequently, the movable scanning element 12, shown
in FIG. 1, is employed in the camera for the purpose of increasing
the field of view of the optical path 28 so that radiation energy
from the entire scene is scanned, directed or steered into the
focal plane array 24.
[0034] The movable scanning element 12 directs or scans emanated
radiation energy from all points 16 of the scene 14 into the focal
plane array 24, so that the emanated radiation energy from the
entire scene 14 is detected and used to create an image. Of course,
if the focal plane array 24 had enough channels positioned at the
appropriate locations so that the inherent field of view of each
antenna of each channel received energy from the entire scene 14,
and there was some amount of overlap between the fields of view
inherently obtained from the antennas of the channels, it would not
be necessary to use the movable scanning element 12. However
because of practical size and cost considerations, the movable
scanning element 12 will typically be employed in the camera
10.
[0035] One type of movable scanning element 12 employs a wedge
shaped refractive element 40 which is rotated about an axis 42. The
wedge shaped characteristic of the element 40 increases the field
of view of the optical path 28 by refracting the energy from a
wider field of view. Consequently, energy from all points 16 within
the scene is scanned into one or more of the channels 26 of the
focal plane array 24. The increase in the field of view achieved by
the scanning element 12 results because of the refraction
characteristics of the wedge shaped element 40, and by rotation of
the wedge shaped element 40 around the axis 42.
[0036] The rotation of the wedge shaped element 40 creates a
scanning path or trajectory in the form of a circle, as shown in
FIG. 4. Five circles 44, 46, 48, 50 and 52 are illustrated in FIG.
4, and each circle 44, 46, 48, 50 and 52 illustrates the scanning
path or trajectory of points 16 in the scene 14 from which five
separate channels 26 of the focal plane array 24 (FIGS. 1 and 2)
receive radiation energy emanated from the scene 14. One channel of
the focal plane array receives radiation energy emanated from those
points 16 of the scene 14 which are illustrated by a single circle.
Some of the circles 44, 46, 48, 50 and 52 shown in FIG. 4 overlap
with one another, indicating that different channels receive energy
emanated from the same points within the scene 14 where the overlap
occurs.
[0037] FIG. 5 illustrates the overlapping nature of all of the
fields of view of sixty-four circles created by sixty-four channels
of a practically sized focal plane array. The overlapping nature of
all of the fields of view shown in FIG. 5 is the same functionally
as the fewer number of fields of view shown more simplistically in
FIG. 4.
[0038] As is apparent from the overlapping scan patterns shown in
FIGS. 4 and 5, each channel receives radiation emanating from many
points in the scene 14 but none of the channels receive radiation
emanated from all of the points in the scene. The non-overlapping
nature of some of the scanning patterns shown by the circles 44,
46, 48 and 50 in FIG. 4 illustrates that some channels receive
radiation that other channels in the focal plane array do not
receive, and the fact that none of the scanning patterns encompass
the entire scene 14 illustrates that none of the channels receive
radiation from all points in the scene.
[0039] The output value signals for each channel are electrical
signals derived by interaction of conventional components of each
channel 26. The conventional components of a radiometer channel 26'
are shown in FIG. 3. The characteristics of the cavity 38 of the
antenna 30 cause the radiation energy at a desired millimeter
wavelength to set up a traveling wave within the cavity 38. The
radiation energy at the desired millimeter wavelength has the
effect of developing a relatively small electrical signal between
the two elements 32 and 34, and this signal represents the strength
of the radiated energy at the desired millimeter wavelength
frequency. The signal from the antenna elements 32 and 34 is
amplified by a low noise amplifier 54 and the amplified signal is
applied to a mixer 56.
[0040] In the mixer 56, the output signal from the low noise
amplifier 54 is heterodyned with a signal from a subharmonic local
oscillator 58. The result of the heterodyning in the mixer 56 is an
output signal from the mixer 56 which is reduced in frequency
relative to the desired millimeter wavelength frequency to which
the antenna 30 responds, but which retains the energy content
information of the millimeter wavelength signal received by the
radiometer channel 26'.
[0041] The reduced frequency signal from the mixer 56 is applied to
another amplifier 60 which amplifies its magnitude before supplying
the amplified signal to a detector 62. The detector 62 recognizes
the value of the energy content of the signal supplied to it and
supplies an output signal related to the energy content to another
amplifier 64. The amplifier 64 further amplifies the level of the
signal from the detector 62 and supplies the amplified signal to a
sample and hold circuit 66. The signal supplied to the sample and
hold circuit 66 therefore represents the amount of energy at the
desired millimeter wavelength frequency which the radiometer
channel 26' detects.
[0042] The sample and hold circuit 66 responds to a control signal
68 to establish the time at which the energy content-related signal
from the amplifier 64 is to be sampled and held. In response to the
control signal 68, the sample and hold circuit determines the value
of the energy content-related signal supplied to it from the
amplifier 64, and holds that sample value until the next subsequent
control signal 68 causes it to sample another instantaneous value
of the signal supplied by the amplifier 64 at that later time.
[0043] The sample signal which is held and supplied by the sample
and hold circuit 66 is an output value signal 70. The output value
signal 70 represents the quantity of millimeter wave radiation
within a relatively narrow frequency band surrounding a frequency
equal to two times the local oscillator frequency, as detected by
the radiometer channel 26' at approximately the time of the control
signal 68.
[0044] The output value signal 70 from the sample and hold circuit
66 is applied to a multiplexer 72. The multiplexer 72 may be common
to a multiplicity of radiometer channels 26', such as all of the
radiometer channels 26' attached to a single dielectric substrate
36 (FIG. 2). Alternatively, the multiplexer 72 may be common to all
of the channels of the entire focal plane array 24 (FIGS. 1 and 2).
The multiplexer 72 selects the desired output value signal 70 from
each of the radiometer channels 26' in response to a select signal
74, and the selected radiometer output value signal 70 is
thereafter supplied from the multiplexer 72 to a signal processor
76 of the camera 10 (FIG. 1). The signal processor 76 uses the
output value signals 70 from the radiometer channels 26 to
formulate or compose an image 78 (FIG. 1) which corresponds to the
scene 14. The signal processor 76 preferably includes a
conventional digital computer which has been programmed to perform
the actions and to execute the computations described herein.
[0045] Although not specifically shown, a receiver channel is
generally similar to the radiometer channel 26' shown in FIG. 3,
except that in place of the detector 62 a conventional magnitude
and phase detector component is employed. The conventional
magnitude and phase detector component recognizes phase information
between the illuminating signal and the received signal, as well as
the magnitude of the energy content at the particular phase
relationship. The magnitude and phase information is sampled and
held by the circuit 66 in response to the control signal 68, and
the multiplexer 72 delivers the signals containing the magnitude
and phase information to the signal processor 76 (FIG. 1). The
additional phase relationship information allows the signal
processor 76 (FIG. 1) to derive additional knowledge describing the
scene, such the range and the velocity of points within the
scene.
[0046] It is also typical in a receiver channel (not shown) that
the component designated as the local oscillator 58 in FIG. 3 also
performs the role of generating the energy use to illuminate the
scene, as well as a heterodyning function. In such circumstances a
conventional beam splitting device is used to direct part of the
illuminating energy out of the antenna 30 and to allow energy
received from the scene to be transferred through the other
components of the receiver channel to function in the manner
described. Under such circumstances, a signal describing the
illuminating signal is supplied to the magnitude and phase
detection component which replaces the detector 62, so that the
phase information can be derived as well as the magnitude
information.
[0047] As shown in FIG. 1, the signal processor 76 receives
position signals 80 from a position sensor 82 which is associated
with the movable scanning element 12. The position sensor 82
derives signals corresponding to each of the scanning or rotational
positions of the wedge shaped element 40. One technique for
deriving the position signals is to attach a bar code to the
exterior of the rotating wedge shaped element 40, and to read the
bar code directly to obtain rotational position information at each
rotational interval of the wedge shaped element 40. The information
from the position signals 80 correlates to points 16 in each
scanning trajectory of the scene 14, represented by the circles
44-52 (FIG. 4), for each channel 26 of the focal plane array
24.
[0048] The position signals 80 are also used by the signal
processor 76 to coordinate the delivery of the control signals 68
to the sample and hold circuit 66 (FIG. 3). In response to the
control signals 68, the output value signals 70 from each channel
26 are derived at points 16 in the scene 14 which correlate to the
position signals 80. Consequently, each output value signal 70 from
each channel 26 is established and correlated to a point 16 in the
scene 14 by the position signals 80 and by the delivery of the
control signals 68 and the select signals 74 (FIG. 3) in relation
to the position signals 80.
[0049] The signal processor 76 associates the output value signals
70 from points 16 in the scene 14 with individual pixels 84 of a
display 86 in accordance with the position signals 80. An image 78
is formed on the display 86, and the image 78 is formed by
individually controlling the intensity of each of the pixels 84.
The image 78 formed on the display 86 includes a representation of
the scene 14, as well as the object 18 in the scene and the
background 20 of the scene.
[0050] To form the image 78, the signal processor 76 correlates
each output value signal 70 from each channel 26 at each position
of its scanning path, as determined by the position signals 80,
with an individual pixel 84 of the display 86. The signal processor
76 calculates the intensity of each pixel 84 from a sum of the
output value signals 70 associated and correlated with each
individual pixel 84, after the output value signals 70 have been
processed in accordance with the various improvements described
below. The final intensity of the each pixel 84 is dependent on the
contribution from each of the output value signals 70 from each
channel which scans the point 16 in the scene 14 that corresponds
to the pixel 84 in the image 78.
[0051] The correlation of the contributions from the output value
signals 70 to each pixel 84 in the image 78 is facilitated by
generating a table that correlates each specific position signal 80
with each channel 26 that contributes an output value signal to
each pixel 84. Such correlation is illustrated by a pixel position
table shown below. The pixel position table illustrates the form of
entries for a focal plane array 24 with N channels and a wedge
shaped element 40 with L discrete position marks on its periphery,
each of which generates a separate position signal 80. Each entry
in the pixel position table relates to a pixel number lying between
1 and M, where M is the number of pixels in the full image 78.
Computing this table before performing imaging computations, and
storing the table in memory for later access when performing the
imaging computations, is of benefit when composing the full image
78 from the channel output value signals 70 recorded at each
position represented by a position signal 80.
2 Channel 1 Channel 2 . . . Channel N Position 1 Pixel Number Pixel
Number . . . Pixel Number Position 2 Pixel Number Pixel Number . .
. Pixel Number . . . . . . . . . . . . Position L Pixel Number
Pixel Number . . . Pixel Number
[0052] Although the movable scanning element 12 has been described
in conjunction with a rotating wedge shaped element 40, other types
of movable scanning elements may be used. For example, another type
of movable scanning element is a rotating mirror located in the
optical path which is retained at a non-orthogonal angle to its
rotational axis. The non-orthogonal orientation causes reflections
of the radiated energy along a path that is not parallel to the
rotational axis, in a manner which is functionally the same as
creating the scanning path or trajectory achieved by the rotating
wedge shaped element 40.
[0053] The scanning trajectory or path created by the rotating
wedge shaped element 40 is illustrated as circular in FIGS. 4 and
5, but the scanning path need not be circular. A variety of
different scanning paths may be employed depending upon the
geometry and orientation of the focal plane array and the scene
which is to be scanned. For example, linear scans may be used in
certain circumstances. A linear scanning path can be achieved by
using two counter-rotating wedge shaped elements 40 in the optical
path. Any scanning path which involves scanning the scene 14 with
non-zero overlaps between the scanning paths of the individual
channels 26 may be used.
[0054] In general, a single complete scan of the radiation energy
emanating from the scene 14 may be used to create a single image 78
on the display 86. By updating the image 78 with
periodically-executed scans of the scene 14, the images 78 may be
presented on the display 86 in a relatively time-current and
time-updated manner.
[0055] One aspect of the present improvements has resulted from the
discovery that, when the wedge shaped element 40 is rotating and
scanning a scene of a uniform brightness temperature, i.e. a flat
field, the output value signal 70 for each channel 26 of the focal
plane array 24 is dependent on the position of the wedge shaped
element 40. This discovery has led to the realization that output
value signals of the channels become partially independent of the
brightness temperature of the points 16 in the scene 14. The
scene-independent portions of the channel output value signals
which occur as the wedge shaped element 40 accomplishes a full scan
of uniform field scene 14 are described herein as a baseline signal
or curve for that channel. The strength of the baseline signal is
dependent upon the position of the wedge shaped element 40. More
importantly however, the value of the baseline signal may overwhelm
the change in channel output value signals that occurs in response
to brightness temperature contrasts in a typical scene 14.
Consequently, the effect of the baseline signal may eliminate or
reduce contrasts in brightness temperature when scanning the
scene.
[0056] While the exact cause of the baseline signal may not be
fully understood, it is believed that the baseline signal is the
result of a standing wave which occurs between the antenna 30
(FIGS. 2 and 3) of each channel 26 and the refractive surfaces of
the rotating wedge shaped element 40. Because of the angled surface
of the wedge shaped element 40, the distance between the antenna 30
and the refractive surfaces of the wedge shaped element 40 varies
in direct relationship to the rotational position of the wedge
shaped element. This change in distance or geometry has the effect
of changing the phase or reflection characteristics which a
standing wave would undergo between the wedge shaped element 40 and
the antenna 30, at different positions of the wedge shaped element
40. This being the case, other types of movable scanning elements
12, such as a mirror with a non-orthogonally oriented rotational
axis, is also likely to create standing waves and scene-independent
variations in the output value signals from the channels.
[0057] The changing characteristics of the standing wave at
different positions of the wedge shaped element 40 causes the
magnitude of the output value signal 70 from each channel to be
altered in a manner related to the changing values or strengths of
the standing wave. The output value signals 70 from each channel
are thereby varied by an influence which is entirely independent of
the radiation emanating from the scene. The change in value of the
output signal resulting from this effect is undesired because it is
interpreted as a change in emanated radiation from the scene, and
therefore adversely influences the composition or formation of the
image 78 on the display 86.
[0058] It has been discovered that the shape or curve of the
baseline signals is substantially stable from one complete rotation
of the wedge shaped element to the next, and the characteristics of
the baseline signal curve are repeatable during each revolution of
the wedge shaped element over relatively long periods of time. The
dependence of the magnitude of the baseline signal of each channel
on the rotational position of the wedge shaped element 40, and the
substantial consistency of this baseline signal over relatively
long periods of time, permits the baseline signal to be identified
and valued, and then subtracted from the output value signals from
the channels derived at corresponding rotational positions of the
wedge shaped element. Subtracting the value of the baseline signal
from the output value signals of the channels at each position of
the wedge shaped element has the effect of eliminating the
scene-independent influences on the channel output value signals,
permitting values to be obtained which truly relate to the
brightness temperatures of the points 16 of the scene 14.
[0059] Formulating the value of the baseline signal involves
recording the output value signals from each channel over a
selected number v of consecutive revolutions of the wedge shaped
element while scanning a scene having a flat field of uniform
brightness points. Averaging the output value signals at each
position over the selected number of consecutive revolutions yields
a curve of baseline signals with any noise related variation
suppressed for each channel.
[0060] The computation of the baseline signals for each channel is
as follows. The output value signal u (70) from each channel i at
rotational position k of the wedge shaped element in scan number n
is denoted by u.sub.ik (n). The dependence on scan number n is
equivalent to a dependence on time. The estimated baseline values
B.sub.ik for each channel at the positions are set forth by
Equation (1): 6 B ik = 1 v n = 1 v x ik ( n ) , 1 i N , 1 k L ( 1
)
[0061] In Equation (1), N is the number of channels, and L is the
number of rotational positions of the wedge shaped element 40. It
has been found that the use of .nu.=100 revolutions creates
sufficient data for forming values of the baseline signal
characteristics for each channel.
[0062] Once the baseline curve for each channel has been developed,
the correction of the output value signals 70 for each channel is
performed by subtracting the baseline values from the observed
output value signals. Subtraction yields the signal x.sub.ik that
is substantially free from the scene-independent and undesired
effects of rotational position of the wedge shaped element, as
shown by Equation (2):
x.sub.ik(n)=u.sub.ik(n)-B.sub.ik, 1.ltoreq.i.ltoreq.N,
1.ltoreq.k.ltoreq.L. (2)
[0063] In Equation (2), the value of the baseline curve for each
channel i at the position k is represented by the value
B.sub.ik.
[0064] By identifying and measuring the value of the baseline curve
for each channel at each position of the wedge shaped scanning
element 40, and thereafter subtracting the value of the baseline
curve from the output value signals from each channel at each
position, the signals used by the signal processor 76 in creating
the image 78 represent the differences and contrasts in the
brightness temperature of the points 16 within the scene 14.
Consequently, the image 78 is more accurate since it is no longer
adversely influenced by the scene-independent influences.
[0065] The baseline-subtracted channel samples or observations
x.sub.ik are used to create a set of new values {circumflex over
(x)}.sub.ik that are used in performing other improvements
described below. Those improvements result in the channel output
value signals being normalized more effectively. The improved
normalization enhances the quality of the image 78 by avoiding
reductions in contrast between the pixels 84 arising from effects
other than actual differences in brightness temperature between
corresponding points 16 of the scene 14.
[0066] A previously known millimeter wave image composition
technique has recognized the fact that each channel has different
response characteristics. The previously known technique has
attempted to scale or normalize the output value signals from the
channels in relation to the differences in gain or amplification of
each of the channels. While this previously known normalization
technique has been partially effective, it has not been completely
accurate because it has failed to take into account certain factors
which are important for good contrast in millimeter wave imaging.
For example, the previously known technique has assumed that each
channel encounters the same mean scene brightness temperature. In
reality, each channel scans only a portion of the scene, as shown
by FIG. 4, and therefore each channel does not encounter the same
mean scene brightness temperature because each channel does not
scan the entire scene 14. Using the same mean scene brightness
temperature as a basis for normalizing the response characteristics
of all of the channels cannot achieve complete and accurate
normalization because the normalizing factor does not apply equally
to the response characteristics of each channel. The implication of
this simplifying assumption in previously known normalizing
techniques is that there has been non-optimal accounting for the
drift in offset values of each channel over time. The offset value
refers to a characteristic output value signal level of each
channel in response to a known input signal. The output value
signal level from each channel will be slightly different in
magnitude in response to a known uniform input signal applied to
all of the channels. Lastly, despite recognizing that individual
channel gain may be a basis for normalization, the previously known
normalizing technique failed to fully compensate for all the
factors which may influence inter-channel differences in gain as is
described below.
[0067] The previously known normalizing technique relates the
voltage u.sub.i of the output values signal 70 from the channel i
(26) to the gain of the channel g.sub.i, the scene brightness
temperature s.sub.i, the channel receiver noise temperature
T.sub.i.sup.R, and the offset voltage u.sub.o,i as follows in
Equation (3):
u.sub.i=g.sub.i(s.sub.i+T.sub.i.sup.R)+u.sub.o,i (3)
[0068] Equation (3) is true only as long as a random noise portion
of the output value signal from the channel is neglected. The more
precise equation for u.sub.i must include a noise voltage term
y.sub.i in the right hand side of Equation (3), where y.sub.i is a
random variable with mean zero and standard deviation denoted by
.sigma..sub.i. Neglecting the noise term y.sub.i is acceptable for
known prior art conclusions about counteracting the effect of the
drift in channel gain, and about removing the dependence on channel
offset. However, a problem occurs when the above incomplete
Equation (3) is implicitly used to develop an expression for the
variance (.sigma..sub.i.sup.V).sup.2 of the output value signals of
the channels i when viewing a scene containing a brightness
temperature distribution with variance (.sigma..sup.S).sup.2. The
problem is represented by the following erroneous Equation (4):
[0069] Equation (4) implies that the standard deviation of the
channel output when viewing a scene with non-zero contrast is
linearly proportional to the gain of the channel, and this leads to
using .sigma..sub.i.sup.V to normalize the inter-channel difference
in gains.
[0070] However, were the noise term y.sub.i taken into account, the
correct expression for the channel output variance would be shown
by the following Equation (5):
(.sigma..sub.i.sup.V).sup.2=g.sub.i.sup.2(.sigma..sup.S).sup.2+(.sigma..su-
b.i).sup.2 (5)
[0071] Since each channel has a thermal noise characteristic and
some uncorrelated "backend" noise of standard deviation
.sigma..sup.Backend, an expression for the noise variance
.sigma..sub.i in terms of the gain of the channel g.sub.i, the
channel receiver noise temperature T.sub.i.sup.R, the pre-detection
bandwidth .beta. and the time constant associated with the
post-detection bandwidth .tau., and .sigma..sub.Backend can be
written as follows in Equation (6): 7 ( i ) 2 = ( g i T i R ) 2 + (
Backend ) 2 ( 6 )
[0072] Backend noise results from the slight differences that arise
from quantizing analogue values into digital values, and
environmental noise from other adjacent electrical components.
[0073] Equation (6) permits the standard deviation of the output
value signals of the channels to be written as shown in Equation
(7): 8 i V = g i 2 [ ( S ) 2 + ( T i R ) 2 ] + ( Backend ) 2 ( 7
)
[0074] Equation (7) reveals the impact of neglecting to consider
the noise standard deviation in using .sigma..sub.i.sup.V to scale
the channel outputs. Consider a channel with such low gain g.sub.i
that its .sigma..sub.i.sup.V value is more or less equal to the
backend noise standard deviation .sigma..sup.Backend. After
multiplication by the reciprocal of .sigma..sub.i.sup.V (i.e. the
scaling operation proposed in the known prior art), the output
value signals of that channel would dominate over other channels
with appreciable gain (and, therefore, higher channel standard
deviation). Thus, the summation of values from different channels
during image composition can potentially create anomalous effects
in the image corresponding to undesirable domination of the pixels
along the scanning path trajectory of extremely low gain
channels.
[0075] Even if the .sigma..sup.Backend related issue were not
present, the channel receiver noise temperature T.sub.i.sup.R that
appears in the thermal noise of the channel varies from channel to
channel, and is independent of channel gain g.sub.i. This implies
that only under the additional special condition of the same value
of T.sub.i.sup.R for all channels would the prior art normalizing
technique work as intended. In reality the channel noise
temperatures vary significantly from one channel to the other,
despite efforts to thermally stabilize each of the channels.
[0076] To avoid these deficiencies in prior art normalizing
techniques, .sigma..sub.i.sup.V is not used to compensate the
inter-channel differences in gain. Instead, the present invention
relies on the results of hot and cold load calibration experiments
to determine channels gains. Experience has shown the gain to be
stable enough for long periods of time to scale the gain of the
channel outputs.
[0077] The improvements in normalization also forego the assumption
that every channel in the focal plane array encounters the same
mean scene brightness temperature during a complete scan of the
scene 14. While this improves the degree of normalization and the
degree of contrast and quality of the image produced, it becomes
necessary to develop an entire set of mean scan brightness
temperatures, one mean scan brightness temperature for each channel
26. The technique employed involves estimating the mean scan
brightness temperature for each channel. To do so it is necessary
to provide a mathematical specification of the image formation
process.
[0078] The mathematical expressions for the image intensity value
I(m) of a given pixel m are statements of the result of combining
or adding signals related to and derived from the channel output
value signals 70 (FIG. 3) made during one complete scan. The
mathematical expressions are in terms of normalized observations,
obtained from the channel output value signals 70 obtained by
applying a normalizing or flat fielding technique, either the prior
art technique described previously, or the improved technique
constituting part of the present application described below.
[0079] If every channel 26 in the focal plane array 24 had the same
noise response characteristics, which is not the case, the
normalized output value signals 70 could be obtained from the
different channels without first weighting or adjusting them in any
manner. Such a circumstance would result in the following Equation
(8) for the image intensity l(m) at pixel m: 9 I ( m ) = j = 1 N l
= 1 L jlm x ^ jl j = 1 N l = 1 L jlm ( 1 m M ) . ( 8 )
[0080] However, the different channels have different noise
temperatures and different offsets, and therefore, different noise
standard deviations, even after normalizing by the gain of the
channel. Greater weight should be given to the channels that are
less noisy in composing the image 78. The best weight to apply to
the contribution from channel j is inversely proportional to the
noise standard deviation, {circumflex over (.sigma.)}.sub.j, of the
normalized samples or observations. This follows from the well
known principle that when forming the weighted sum of two or more
random variables which have the same means but different variances,
the variance of the sum is minimized when the weighting is
inversely proportional to the standard deviation of the individual
variables. The application of this principle yields the following
Equation (9) for the final image intensity of each pixel: 10 I ( m
) = j = 1 N l = 1 L jlm x ^ jl ^ j j = 1 N l = 1 L jlm ^ j , 1 m M
. ( 9 )
[0081] The image intensity I(m) is the estimate of the relative
brightness temperature of the point 16 in the scene 14 represented
by the pixel m. With the notation and image intensity Equation (9),
the improved flat fielding or normalizing can be described.
[0082] The improved flat fielding technique involves calculating
the mean scan temperatures for each channel, .theta..sub.1,
.theta..sub.2, . . . , .theta..sub.N, by setting up a system of N
simultaneous linear equations expressing relationships between the
N unknowns .theta..sub.1, .theta..sub.2, . . . .theta..sub.N. The
solution to these simultaneous linear equations yields the values
of .theta..sub.1, .theta..sub.2, . . . , .theta..sub.N that permit
computation of the improved flat fielded image intensities.
[0083] The normalized samples or observations {circumflex over
(x)}.sub.ik from each channel i (26) are expressed in terms of the
output value signals (70) x.sub.ik and the unknown .theta..sub.i in
the following Equation (10): 11 x ^ ik = x ik - x _ i g i + i , 1 i
< N , 1 k L . ( 10 )
[0084] For the normalization represented in Equation (10), the
associated noise standard deviation {circumflex over
(.sigma.)}.sub.i, is related to the raw standard deviation
.sigma..sub.i in the following Equation (11): 12 ^ i = i g i 1 i N
. ( 11 )
[0085] Substituting in the preceding two Equations (10) and (11),
in the expression for the image intensity given in Equation (9),
results in the following Equation (12): 13 I ( m ) = j = 1 N l = 1
L jlm ( x jl - x _ j g j + j ) g j j j = 1 N l = 1 L jlm g j j , 1
m M . ( 12 )
[0086] .theta..sub.1, .theta..sub.2, . . . , .theta..sub.N in
Equation (12) are, as yet, unknown. The correct image intensities
l(m) are also unknown. However, Equation (12) makes explicit the
dependence of the image intensities on the unknown .theta..sub.1,
.theta..sub.2, . . . , .theta..sub.N, and leads to the central
concept of the improved flat fielding technique: For each channel,
impose the consistency condition that the unknown mean scan
temperature encountered by each channel be equal to the average of
the intensities of those image pixels which that channel
contributes to, relying, for the calculation of the image
intensities average, on the expression for the intensities in terms
of the unknown mean scan temperatures.
[0087] These consistency requirements provide a way of deriving as
many simultaneous linear equations as the number of unknowns.
Consider the circular path or trajectory of the energy scanned into
any channel as it traverses the scene 14 during a scan. An
expression for the mean of the brightness temperatures along this
path is expressed in terms of the l(m) values along the path. For
channel i, this expression is set forth in Equation (13) as
follows: 14 Mean brightness of path from image = k = 1 L m = 1 M
ikm I ( m ) k = 1 L m = 1 M ikm = 1 L k = 1 L m = 1 M ikm I ( m ) (
13 )
[0088] Equation (13) makes use of the fact that the denominator in
the middle expression sums to L. The requirement is next imposed
that this mean scan brightness temperature, which is a function of
all the channel mean brightness temperatures, be equal to
.theta..sub.i, the mean scan brightness temperature encountered by
the particular channel whose trajectory was traversed in computing
the above path mean. This strategy yields one equation per channel
for each of the N channels in the focal plane array, as shown in
Equation (14): 15 1 L k = 1 L m = 1 M ikm I ( m ) = i , 1 i N . (
14 )
[0089] Substituting in Equation (14), the expression for l(m) given
in Equation (12) obtains the following linear system of equations
(Equations (15)) defining the unknown per channel mean relative
brightness temperatures: 16 1 L k = 1 L m = 1 M ikm j = 1 N l = 1 L
jlm g j j j j = 1 N l = 1 L jlm g j j - L i = - k = l L m = 1 M ikm
j = 1 N l = 1 L jlm ( x jl - x _ j j ) j = 1 N l = 1 L jlm ( g j j
) , 1 i N . ( 15 )
[0090] It is useful to write Equation (15) using matrix notation as
follows in Equation (16): 17 A [ 1 N ] = b . ( 16 )
[0091] In Equation (16), A on the left hand side, is an N.times.N
matrix of coefficients, and b on the right hand side is an
N.times.1 column vector. Each of the rows of A sum to zero, making
A a singular matrix. This is consistent with the fact that the mean
scan brightness temperatures are relative, permitting them to be
estimated only up to an arbitrary additive term. So, with no loss
of generality, one of the .theta..sub.1, .theta..sub.2, . . . ,
.theta..sub.N can be set to an arbitrary value. It is convenient to
set .theta..sub.N to zero. This permits rewriting the left hand
side of Equation (16) as follows: 18 A [ 1 N - 1 0 ] = [ A 1 A N -
1 A N ] Matrix A expressed in terms of its Columns A 1 through A N
[ 1 N - 1 0 ] = [ A 1 A N - 1 ] Matrix of Size N .times. ( N - 1 )
[ 1 N - 1 ] = A ' [ 1 N - 1 ] ( 17 )
[0092] In Equation (17) A' has been introduced to denote the
N.times.(N-1) matrix that results from deleting the N-th column
vector of matrix A. Thus, the system of equations to be solved is
the following Equation (18): 19 A ' [ 1 N - 1 0 ] = b . ( 18 )
[0093] The positing of the consistency conditions among the various
per channel mean scan brightness temperatures and the subsequent
derivation of the linear system given in Equation (18) is
implemented to achieve improved normalization or flat fielding. The
actual solution of the equations is carried out using a
conventional computational method of solving linear systems.
Nevertheless, there are a few important details that must be
considered when computing the solution.
[0094] A comparison of the left hand sides Equations (15) and (18)
reveals that the elements of the matrix of coefficients A' are
independent of the channel output value signals (70). This enhances
the speed at which the computations are executed. If this was not
the case, it would be necessary to undertake the high computational
burden of computing the pseudo-inverse of an N.times.(N-1) matrix
each time the flat fielding normalizing factors were computed.
Fortunately, this burden is reduced by performing an inversion of
matrix A' prior to performing the computations to obtain the flat
fielding normalizing factors, storing in memory the resulting
pseudo-inverse (A').sub.inv, and using the stored (A').sub.inv
during the actual flat fielding operation. The singular value
decomposition of A' is used to calculate its pseudo-inverse,
(A').sub.inv.
[0095] A comparison of the right hand sides of Equations (15) and
(18) shows that the elements of the column vector b depend on the
channel output value signals, and are sums of the intensities of an
intermediate image I'(m) that can be defined by the following
Equation (19): 20 I ' ( m ) = j = 1 N l = 1 L jlm ( x jl - x _ j j
) j = 1 N l = 1 L jlm g j j , 1 m M . ( 19 )
[0096] It is convenient to compute and temporarily store I'(m)
before proceeding to the estimation of .theta..sub.1,
.theta..sub.2, . . ., .theta..sub.N.
[0097] The right hand side column vector b of Equation (18) is
calculated anew at each scan to obtain the estimates of the channel
mean scan temperatures. The .theta..sub.1, .theta..sub.2, . . . ,
.theta..sub.N-1 are obtained by post-multiplying the stored
pseudo-inverse of matrix A', i.e. (A').sub.inv, by the most
recently formed b as shown in the following Equation (20): 21 [ 1 N
- 1 ] = ( A ' ) inv Precomputed pseudo - inverse b , And , of
course N = 0. ( 20 )
[0098] The estimated .theta..sub.1, .theta..sub.2, . . . ,
.theta..sub.N are then used to update the intermediate image I'(m)
to obtain the final flat fielded image I(m), 1.ltoreq.m.ltoreq.M,
that is described by Equation (12).
[0099] It has been discovered that even when using the normalized
channel output value signals, anomalous effects appear in the image
78. The anomalous effects result from differences in channel noise
characteristics, which can not be predicted or calculated.
Nevertheless, the present improvements further enhance the quality
of the image 78. These improvements are better understood by
reference to an image composition technique employing only direct
summation of the normalized channel output value signals, as
described below in Algorithm 1.
[0100] The image 78 is derived from the output value signals 70 of
the individual channels 26 at the end of each scan of the scene. In
the simple but unrealistic situation where all the channels of the
focal plane array had the same gain, offset, and noise temperature,
the image composition would be accomplished by summing the output
values signals 70 as contributions to each image pixel, and
normalizing each sum by the number of contributions received.
Assuming the channels are sampled at each discrete position of the
wedge shaped element 40, the inputs into this computation are the L
number of subframes, with each subframe containing N number of
channel output value signals 70 (each an observation), and the
measured position of the wedge shaped element 40 represented by the
position signals 80. The channel observations or samples are
denoted by with the first subscript, i, specifying the channel
number and the second subscript, k, the position of the wedge
shaped element. (1.ltoreq.i.ltoreq.N, 1.ltoreq.k.ltoreq.L). A
precomputed table of pixel positions is exemplified by the table
set forth above.
[0101] In a radiometer based millimeter wave imaging camera, the
following Algorithm 1 is applied to compose the complete image 78,
whose intensity at each pixel m is denoted by I(m):
3 Algorithm 1: Basic Unweighted Image Composition 1. Compute the
pixel position table. 2. Initialize the image display I(m) to zero
for 1 .ltoreq. m .ltoreq. M. 3. Initialize an array of contribution
counts C(m) to zero for 1 .ltoreq. m .ltoreq. M. 4. For each
position k between I through L of the wedge shaped element, and for
the contribution from each channel number i between 1 through N,
Look up the pixel number m corresponding to position k and channel
i from the precomputed pixel position table. I(m) = I(m) +
x.sub.ik. C(m) = C(m) + 1. 5. For each m between 1 and M, 22 I ( m
) = I ( m ) C ( m )
[0102] If each channel of the focal plane array had the same noise
characteristics and their different gains and offsets had been
normalized, Algorithm 1 could be used to compose the image 78,
using the modified channel observations {circumflex over
(x)}.sub.ik obtained from the original channel observations
x.sub.ik by using flat fielding. However, each channel does not
have the same noise characteristics. Therefore, even after the
output value signals of each channel have been normalized to
account for gain and offset differences, preferably using the
improved flat fielding technique described above, the channels are
likely to still differ in their noise characteristics. This is
confirmed by noise measurement experiments and direct measurements
of the standard deviations of noise from each of the channels.
[0103] The improved aspect of the present invention is to give
greater weight to those output value signals 70 from channels 26 of
the focal plane array 24 which are less noisy compared to the
output value signals 70 from channels 26 which are more noisy, as
discussed above in conjunction with Equation (9). The best weight
to apply to the contribution from channel i is inversely
proportional to the noise standard deviation, {circumflex over
(.sigma.)}.sub.i, of the normalized channel observations. This
follows from the well known principle that when forming the
weighted sum of two or more random variables which have the same
means but different standard deviations, the standard deviation of
the sum is minimized when the weighting is inversely proportional
to the standard deviation of the individual variables.
[0104] Employing this weighting concept in composing the millimeter
wave image 78 involves calculating a weighted summation of the
normalized channel observations. The weight to be given to the
normalized channel observations is the reciprocal of the normalized
noise standard deviation 23 1 ^ i ,
[0105] for 1.ltoreq.i.ltoreq.N . So, the improvement in the image
composition algorithm requires an additional set of inputs (the
weights) to be applied to the normalized observations {circumflex
over (x)}.sub.ik, as set forth in the following Algorithm 2:
4 Algorithm 2: Channel Weighting Based Image Composition 1. Compute
the pixel position table. Measure the channel noise standard
deviations. 2. Initialize the image array I(m) to zero for 1
.ltoreq. m .ltoreq. M. 3. Initialize an array of contribution
counts C(m) to zero for 1 .ltoreq. m .ltoreq. M. 4. For each wedge
position k between 1 through L, and for each channel number i
between 1 through N, Look up the pixel number m corresponding to
wedge position k and channel i from the precomputed position table.
24 I ( m ) = I ( m ) + x ^ ik ^ i . C ( m ) = C ( m ) + 1 ^ i . 5.
For each m between 1 and M, 25 I ( m ) = I ( m ) C ( m )
[0106] Mathematically, the image composed by use of the direct
summation algorithm, Algorithm I set forth above, can be expressed
by Equation (8). Specifying the .delta..sub.ikm in Equation (8) is
equivalent to specifying the scanning pattern in the form
exemplified above in the pixel position table. Thus,
.delta..sub.ikm can be precomputed for 1.ltoreq.i.ltoreq.N,
1.ltoreq.k.ltoreq.L, and 1.ltoreq.m.ltoreq.M.
[0107] In contrast to Equation (8), the result of using the
weighted summation image composition algorithm, Algorithm 2 set
forth above, can be mathematically expressed in Equation (9).
[0108] The weighted summation image composition Equation (9), leads
to the suppression and substantial elimination of undesired
anomalous effects created by excessively noisy channels.
[0109] After having eliminated the effects of baseline signals, and
upon using the flat fielding technique for channel normalization
based on gain measurements obtained by hot and cold load
calibration experiments, and upon weighting the contributions of
each channel in the summation of the values for each pixel of the
image in such a way that the contributions from the more noisy
channels are diminished while the contributions from the less noisy
channels are enhanced, the improved composition of the image 78 may
proceed according to Algorithm 3 described below. Algorithm 3
illustrates the sequence of carrying out the steps of image
formation in a radiometer based millimeter wave imaging camera.
Algorithm 3 incorporates baseline subtraction, flat field
normalizing, and channel weighting as described above.
5 Algorithm 3: Image Formation with Baseline Subtraction, Channel
Weighting and Flat Fielding 1. Compute channel standard deviations
.sigma..sub.i, channel gains g.sub.i, and the matrix (A').sub.inv
from calibration experiments channel observations. Compute the
pixel position table. Compute the baseline signal B.sub.ik, 1
.ltoreq. i .ltoreq. N, 1 .ltoreq. k .ltoreq. L. 2. Initialize the
image array 1(m) to zero for 1 .ltoreq. m .ltoreq. M. 3. Initialize
an array of contribution counts C(m) to zero for 1 .ltoreq. m
.ltoreq. M. 4. Subtract the baseline: For each wedge position k
between I through L, For each channel number i between 1 through N,
x.sub.ik = u.sub.ik - B.sub.ik. 5. For each channel number I
between 1 through N, 26 Compute the channel mean : x _ ik = k = 1 L
x ik . 6. For each wedge position k between 1 through L, For each
channel number i between 1 through N, Look up the pixel number m
corresponding to wedge position k and channel i from the
precomputed position table. 27 I ( m ) = I ( m ) + x ik - x _ i i .
C ( m ) = C ( m ) + g i i . 7. For each channel number i between 1
through N, b.sub.i = 0. For each wedge position k between 1 through
L, Look up the pixel number m corresponding to wedge position k and
channel i from the precomputed position table. 28 b i = b i - I ( m
) C ( m ) . 8. Compute the vector of channel-wise mean scan
temperatures 29 [ 1 N - 1 ] = ( A ' ) inv [ b 1 b N ] , and N = 0 ,
9. For each wedge position k between 1 through L, For each channel
number i between 1 through N, Look up the pixel number m
corresponding to wedge position k and channel i from the
precomputed position table. 30 I ( m ) = I ( m ) + g i i i . 10.
For each m between 1 and M, 31 I ( m ) = I ( m ) C ( m )
[0110] The flowchart shown in FIG. 6 describes a process flow 100
of actions, information and computations involved in obtaining and
computing the enhanced image 78 using the improvements described
above. A convention employed in the flowchart 100 is that those
boxes having straight and perpendicular sides represent a
computation, those boxes having slanted and non-perpendicular
vertical sides represent data that is used once, and those boxes
having curved vertical sides represent data that is used
repeatedly, i.e. persistent data.
[0111] The process flow 100 begins at 102 where observations from
several scans over two scenes of uniform brightness are collected.
One of the two scenes of uniform brightness may be the background
within an enclosure which has been air-conditioned to achieve a
uniform temperature throughout. The second one of the two scenes of
uniform brightness may be established by a black object inserted in
the optical path 28. Scanning two scenes of different brightness
temperatures establishes two different output value signals from
each channel, and these two output value signals are used in
calculating the gain of each channel. In addition or simultaneously
at 103, further observations from scanning a scene of uniform
brightness are also collected to establish a curve of the baseline
signal described above.
[0112] The data obtained from the two scanning steps at 102 and 103
provides the necessary information needed to make the calibration
calculations at 104, to calculate the standard deviations of the
channel output value signals at 108, and to compute the baseline
signal curves at 109. The calibrations at 104 result in
establishing the values of the channel gains at 106. The standard
deviations calculated at 108 proceed in accordance with
conventional standard deviation calculation techniques and result
in the standard deviation values for each channel at 110. The
computation of the baseline at 109 results in the baseline curve
information for each channel at 111.
[0113] At 112, the scanning paths or trajectories of each of the
channels is obtained and correlated to the pixels 84 of the display
86 in the manner described above. Preferably, step 112 is
accomplished by populating and using the pixel position look-up
table described above which correlates the rotational position of
the wedge shaped element 40 and the points 16 in the scene 14 and
the pixels 84 in the image 78 (FIG. 1).
[0114] At 114, the matrix A' is calculated in the manner described
above with respect to Equation (18), and the value of the matrix A'
is established at 116. The pseudo-inverse of (A').sub.inv is next
computed at 118, preferably by using singular value decomposition
as described above, and the pseudo-inverse result is stored at
120.
[0115] The steps 102-120 of the process flow 100 described above
may be executed once and the data resulting from the computations
thereafter stored for use repeatedly over a relatively large number
of subsequent scans when imaging the scene 14. While it may not be
necessary to do so, it may be advisable to repeat the
initialization steps 102-120 periodically during the use of the
millimeter wave imaging camera 10. The initialization steps 102-120
should be performed only after the elements of the millimeter wave
imaging camera have warmed up and reached thermal equilibrium in an
environment where use of the camera is likely to be continued, so
the results from the steps will have the maximum value and
accuracy.
[0116] The remaining steps 122-138 of the process flow 100 are
performed during each scan to create a single image. Each time the
image is updated, the steps 122-138 are preferably again performed.
In this manner, the flat field normalizing factors obtained in
accordance with the present invention will be applied to each image
and each update of that image at the time that the image is created
and updated. Consequently, effects of drift in the offset value
will be immediately and effectively normalized and compensated with
each scan.
[0117] At 122, the output value signals 70 or samples from a scan
of the entire scene are collected as described above. At 121, the
baseline values 111 are subtracted from the output value signals
70, resulting in the creation of baseline subtracted samples
123.
[0118] Using the baseline subtracted samples 123, the intermediate
image I' is formed at 125 by applying the Equation (19), and the
weighting contributions C are obtained by applying step 6 of
Algorithm 3. The result of the computation at 125 is the value of
the intermediate image I' and, weighting contributions C at
127.
[0119] The value of the intermediate image I' and the weighting C
contributions established at 127 are used along with the
information at 112 describing the scanning paths or trajectories of
the channels in a computation of the vector b at 128. The
computation at 128 occurs in accordance with Equations (15) and
(16), and the value of the vector b is established at 130.
[0120] At 132, the stored pseudo-inverse of the matrix obtained
from 120 is post-multiplied by the value of the vector b, obtained
at 130. The computation at 132 results in the estimated values of
the mean scan temperatures for each channel at 134. At 136, the
final image/is computed using the mean scan temperatures obtained
at 134, and the final flat fielded image is produced at 138.
[0121] The production of the final image at 138 obtains the
numerous improvements described, thereby resulting in the creation
of an image 78 having more contrast and resolution, or in the
creation of an image having adequate contrast and resolution from
radiated energy signals with less brightness temperature contrast.
By performing the baseline subtraction before using the camera for
imaging, and by performing the flat fielding and channel weighting
with each scan 122, an improved image results under circumstances
where the response characteristics of the channels can not
otherwise be improved. Many other improvements and advantages will
be apparent upon gaining a complete understanding of the present
invention.
[0122] A presently preferred embodiment of the present invention
and many of its improvements have been described with a degree of
particularity. This description is a preferred example of
implementing the invention, and is not necessarily intended to
limit the scope of the invention. The scope of the invention is
defined by the following claims.
* * * * *